1. Field of the Invention
This invention is related in general to a process of manufacture of thin polymer films by vacuum vapor deposition. In particular, it pertains to the vacuum evaporation and deposition of solid oligomers to form a solid film on a substrate directly without subsequent polymerization or curing reaction.
2. Description of the Related Art
Thin metallic and polymeric films are used on various substrates, such as paper, fabrics, plastic and metal foils, to add or promote desirable properties for particular applications. For example, foils used to preserve food need to have very low permeability to oxygen; the exterior surface of packaging material has to be capable of accepting inks for printing purposes; and packaging materials for electronic products also require a limited amount of conductivity to dissipate electrostatic charges. Similarly, metallic pigment flakes are produced by depositing a thin layer of metal on a web from which the metal is later separated and crushed; therefore, it is important that the web material be suitable for the separation step. To these ends, it is desirable and sometimes necessary to modify the physical properties of polymeric films to improve their suitability for the intended purpose. Preferably, though, the films are directly formed with a composition and molecular structure characterized by the desired properties.
Thin films of metals and polymers are created by deposition onto appropriate substrates by a variety of known processes, most notably through film formation by wet chemistry or vapor deposition. Chemical processes produce soluble thermoplastic as well as insoluble thermoset polymers and involve the use of solvents; thus, film formation is achieved through solvent diffusion and evaporation. As a result, these processes require relatively long residence times and the undesirable step of handling solvents.
Vapor deposition processes involve the evaporation of a liquid monomer in a vacuum chamber, its deposition on a cold substrate (referred to in the art as "cryocondensation"), and the subsequent polymerization by exposure to electron beam or ultraviolet radiation. As illustrated schematically in FIG. 1, the liquid monomer from a supply reservoir 12 is fed through a capillary tube 14 and an atomizer 16 into the heated evaporator section 18 of a vacuum deposition chamber 10 where it flash vaporizes under vacuum. The resulting monomer vapor is then passed into a condensation section 20 of the unit where it condenses and forms a thin liquid film upon contact with the cold surface of an appropriate substrate, typically a rotating drum 22. A metal vaporization unit 24 may also be used to deposit in line a thin metal layer on the drum 22 for multilayer deposition. The liquid deposited film is then cured by exposure to an electron beam or ultraviolet radiation source 26. Since the ultimate objective is the formation of solid films, the initial liquid monomer must be capable of polymerization and contain enough reactive groups to ensure that a sufficiently large polymeric molecule results and yields a solid product.
This conventional approach of utilizing a polymerizable monomer as the raw material for thin-film forming processes has been followed over the years because it is not possible to vaporize the final polymeric product under the range of operating conditions of a commercially viable vapor deposition chamber (typically, 10.sup.-3 to 10.sup.-6 torr and 70.degree. C.-350.degree. C.). Thus, the practice in the industry has been to identify or develop polymers having specific characteristics deemed advantageous for a particular film application. A solid thin film of the polymer is then formed on a target substrate by evaporating the corresponding liquid monomer, cryocondensing it as a monomer in liquid form and polymerizing or cross-linking it to reach the required molecular weight to ensure its solidification. Many variations of this basic approach have been developed for particular applications, but all prior-art vacuum deposition processes involve the formation of a solid film by polymerization of a liquid monomer evaporated under vacuum or atmospheric conditions and recondensed on a cold surface to obtain the desired film characteristics.
This approach has several inherent constraints that often produce disadvantages. Most importantly, the selected monomers have to include reactive moieties, such as acrylate and vinyl groups, in order to enable polymerization and produce a solid, usable film after condensation. Thus, the resulting polymeric film necessarily contains reactive groups that give the film reactive characteristics that may be undesirable for particular uses. For example, the oxygen atoms in acrylate groups gives the film adhesion properties that prevent it from being used as a release coating.
Because thin-film forming by vapor deposition is commercially preferable, considerable research has been conducted to develop processes for improving the properties of thin films obtained by polymerization of vacuum deposited monomers. For example, U.S. Pat. No. 5,681,615 discloses a method for incorporating a salt of desirable characteristics into the polymeric film. The salt is dissolved in the initial liquid monomer, which is then vaporized at a temperature below the decomposition and polymerization temperatures of the composite material and cryocondensed according to conventional practice.
In U.S. Pat. No. 5,902,641, a further improvement is introduced by vaporizing a two-phase mixture of a liquid monomer and insoluble solid particles, thereby producing a homogeneous vapor mixture of the two components. The mixture is then cryocondensed and the monomer is polymerized or cross-linked according to standard procedures to yield a thin film that contains the solid component in uniform, homogeneous distribution. Thus, the process enables the manufacture, for example, of doped polymeric films for the electronic industry.
The main drawback of all prior-art vapor-deposition techniques is the fact that they require polymerization or cross-linking of the liquid-film monomer formed by vacuum evaporation and cryocondensation. Thus, the presence of residual reactive groups in the final product is unavoidable.
Therefore, there is still a need for an approach to thin-film polymer manufacture that makes it possible to produce films without the inherent disadvantages that result from polymerization and/or cross-linking. This invention is directed at a process for achieving this objective by judiciously selecting appropriate oligomers as the starting material for the manufacture of thin films.